1. Field of the Invention
The present invention relates to gradient-index (GRIN) lenses and composite optical elements, optical circuits, optical integrated circuits and so forth utilizing the GRIN lenses.
2. Description of the Related Art
The conventional lenses are generally convex lenses which have spherical or other curved surfaces. In contrast to them, gradient-index lenses, as illustrated in FIG. 1, are made of glass or polymeric material provided with a refractive index profile. They are being manufactured and sold with a brand name of “SELFOC lens” for instance. The lenses of this type are characterized not only by their capacity for miniaturization but also by their easy incorporation into collimating optical systems or confocal optical systems because, for example, of their capacity for focusing on the end surface of the lens. For these advantages, gradient-index lenses are being widely used in the optical information processing area, including optical systems of optical fiber communication or scanners.
As an example of such gradient-index lenses, Japanese Paten Application Laid-Open No. Sho63-273801 discloses one which comprises multiple layers with different refractive indices. With this lens, a desired refractive index profile is obtained by laminating the crystals of ZnSSe or the like by appropriately changing the composition ratios by an MBE (Molecular Beam Epitaxy) or similar method. However, the lens as disclosed in this reference poses a problem of increased errors in the peripheral, or edge, parts of the lens, which are subject to greater variation in refractive index when the layers are each about 10 μm thick. This, in turn, leads to a problem of multiple points of focus due to the occurrence of sub-spots. Although it is not yet fully clarified, the cause of the occurrence of sub-spots is assumed to be as follows. In the middle part of the lens, the changes in refractive index are relatively small, and the physical distances over which light passes are short. In the peripheral parts of the lens, however, the changes in refractive index are large, and the physical distances over which light passes are long. As a result, step approximation of the refractive index profile in the edges of the lens produces greater effect on focusing of light, thus creating an actual focal length different from design. In this manner, light beams passing through the peripheral parts of the lens are assumed to converge on sub-spots.
On the other hand, the lens disclosed in Japanese Paten Application Laid-Open No. Sho60-84501 achieves an improved focusing of light by employing such design that the layer thickness is made thinner in the periphery of the lens to create a Fresnel profile. Yet the method proposed in this reference requires a design of the refractive index profile of the lens to form a diffraction grating, which places restrictions on the optical characteristics of an optical element, such as its dimensions, focal length and wavelengths to be used. Moreover, it is difficult to manufacture the lens of this type since the layer thickness must be controlled over the wide range thereof.
Related Art List
    (1) Japanese Patent Application Laid-Open No. Sho63-273801.    (2) Japanese Patent Application Laid-Open No. Sho60-84501.